The method for objectively evaluating an image quality of an image that has been transmitted (hereinafter, referred to as a transmitted image), generally speaking, is classified into three kinds. A first evaluation method is a method for directly comparing an original image with the transmitted image, thereby to evaluate the image quality. A second evaluation method is a method for evaluating the image quality only from the transmitted image. A third evaluation method is a method for employing a feature quantity of the original image, thereby to evaluate an image quality.
ITU-T (Telecommunication Standardization Sector of International Telecommunication Union) recommends a specific method associated with the first evaluation method as described in Non-patent document 1. The first evaluation method, when trying to evaluate the image quality of the transmitted image, necessitates an original image (the image that has not been transmitted) in a receiving terminal side for evaluating the image quality. However, it is impossible to transmit the original image to the receiving terminal side with the image quality thereof kept intact due to an upper-limit of a communication band of a transmitting path. Thus, the image quality of the transmitted image cannot be evaluated by employing the first evaluation method in the receiving terminal side.
Contrarily hereto, the second evaluation method, so to speak, is an ideal method because the receiving terminal side can evaluate the image quality by itself. However, even though the receiving terminal side tries to evaluate the image quality of the transmitted image by employing the second evaluation method, its realization is difficult because it is hard to separate the original feature of the image from the feature generated due to image quality degradation.
Finally, in the third evaluation method, transmitting the feature quantity of the original image to the receiving terminal side enables the receiving terminal side to separate the original feature of the image from the feature generated due to the image quality degradation. Thus, if an appropriate feature quantity can be transmitted, it can be expected to evaluate the image quality of the transmitted image at a high precision.
In Patent-document 1, the transmitted image quality monitoring device for transmitting the feature quantity of the original image, thereby to evaluate the image quality of the transmitted image, which employs the third evaluation method, is described. The transmitted image quality monitoring device described in the Patent document 1 divides an input image into blocks of an arbitrary size, subjects video within the block to an orthogonal transformation, extracts an arbitrary frequency component value subjected to the orthogonal transformation, and transmits it as feature quantity, thereby to estimate the image quality degradation from a difference of the feature quantity between the original image and the transmitted image. Additionally, the patent document 1 discloses the method for multiplying the video within the block by a sequence (PN sequence), in which +1 and −1 occur at random, before the orthogonal transformation in order to equally disperse an influence due to degradation upon each frequency component. Further, it also discloses the method for, after the orthogonal transformation, multiplying the video within the block by the PN sequence in order to cope with the locally degraded image.    Patent document 1: JP-P2003-9186A    Non-Patent document 1: ITU-T Recommendation, “J.144 Objective perceptual video quality measurement techniques for digital cable television in the Presence of a full reference”, ITU-T recommendation, 2004.